CHERI prep: stave off amplification for Remote routing #266
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mjp41
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Dec 15, 2020
| * pairings will substantially decrease performance and so we prohibit them | ||
| * and use SNMALLOC_ASSERT to verify that they do not exist in debug builds. | ||
| */ | ||
| alloc_id_t alloc_id_and_sizeclass; |
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Can this be a class type? We have some helpers for bitfield extracts, so we could easily provide an abstract interface for getting the two bits.
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So this is pretty contained in a single class already adding another layer of abstraction probably wouldn't buy us much.
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Squeeze some bits out of allocator IDs so that we can land the sizeclass in each Remote object. The intent is that, on StrictProvenance architectures like CHERI, we will be able to route Remote messages through RemoteCache-s without needing to amplify back to read the sizeclass metadata field out of the slab headers.
This removes a bunch of pointer math (which would need amplification) to find and read the sizeclass from slab headers.
When forwarding a Remote message, we can now operate entirely with local state: access to our RemoteCache and the message itself.
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Smuggle the size class in Remote objects, so that we can leave Remote pointers bounded to their underlying allocation and yet forward a Remote message using only local state. That is, we need to avoid going to the {Alloc,Super,Medium,Meta}slab headers unless we're consuming the Remote message back into our free lists.